The present invention relates to optical communication systems, and more particularly to optical fiber lasers and amplifiers having a flattened gain slope for use in communicating optical signals.
Cable television systems currently distribute television program signals via coaxial cable, typically arranged in tree and branch networks. Coaxial cable distribution systems require a large number of high bandwidth electrical amplifiers. For example, 40 or so amplifiers may be required between the cable system headend and an individual subscriber's home.
The replacement of coaxial cable with optical fiber transmission lines in television distribution systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems. Lasers, such as rare earth-doped optical fiber lasers, are used to generate an optical carrier for communicating information signals over the fiber. Optical amplifiers, such as rare earth-doped fiber amplifiers, are used to amplify the signals along the communication path.
Optical fiber amplifiers, and particularly erbium-doped fiber amplifiers, have been proposed for applications in long distance transmission and subscriber loop distribution systems. See, e.g., W. I. Way, et al, "Noise Figure of a Gain-Saturated Erbium-Doped Fiber Amplifier Pumped at 980 nm," Optical Amplifiers and Their Applications, 1990 Technical Digest Series, Vol. 13, Conference Edition, Optical Society of America, Aug. 6-8, 1990, Paper TuB3, pp. 134-137, and C. R. Giles, "Propagation of Signal and Noise in Concatenated Erbium-Doped Fiber Optical Amplifiers," Journal of Lightwave Technology, Vol. 9, No. 2, February 1991, pp. 147-154.
A disadvantage of an erbium-doped fiber amplifier is that its gain spectrum is irregular, with a sharp peak at about 1,532 nm and a broad band with reduced gain at longer wavelengths. In order to overcome this disadvantage, it has been suggested to operate such amplifiers at wavelengths away from the peak gain. However, such operation has a disadvantage in that increased spontaneous-spontaneous beat noise and possible laser action at the peak gain wavelength can occur.
Others have proposed the use of an optical notch filter in an erbium-doped fiber amplifier to shape the gain thereof and effectively suppress the gain spectrum at the peak wavelength. M. Tachibana, et al, "Gain-Shaped Erbium-Doped Fibre Amplifier (EDFA) With Broad Spectral Bandwidth," Optical Amplifiers and Their Applications, 1990 Technical Digest Series, Vol. 13, Conference Edition, Optical Society of America, Aug. 6-8, 1990, Paper MD1, pp. 44-47, and Tachibana, et al, "Spectral Gain Cross Saturation and Hole-Burning in Wideband Erbium-Doped Fibre Amplifiers," Optical Amplifiers and Their Applications, 1991 Technical Digest Series, Vol. 13, Conference Edition, Optical Society of America, Jul. 24-26, 1991, Paper ThB1-1, pp. 104-107. These articles discuss an experimental optical amplifier in which a short length of amplifier fiber was sandwiched between a mechanical grating and a flat plate to induce a resonant coupling at a particular wavelength and thereby shape the spectral gain of the amplifier.
Recent progress has been made in placing gratings in optical fibers by modifying the fiber index of refraction. Examples of processes for forming such gratings can be found in G. Meltz, et al, Optical Letters, Vol. 14, p. 823, 1989 and R. Kashgap, et al, Electronics Letters, Vol. 26, p. 730, 1990. These articles describe the formation of gratings by photorefractive techniques. The gratings disclosed in the articles are perpendicular to the direction in which the optical signal propagates through the substrate containing the grating. When the grating is placed perpendicular to the direction of the lightwave propagation, the light is reflected back upon its original path. Thus, such gratings are used as reflectors.
It would be advantageous to provide a practical method and apparatus for flattening the gain in optical fiber amplifiers and lasers. Such a method and apparatus should be straightforward to implement and provide reliable operation. It would be particularly advantageous to provide an optical fiber amplifier that can handle a plurality of communication signals that are wave division multiplexed onto an optical communication path, wherein the communication path includes a plurality of such amplifiers in a cascaded fashion.
The present invention provides a method and apparatus having the aforementioned advantages.